The present invention relates to a method for monitoring the operation of a doctor assembly in a paper machine, a doctor member belonging to the doctor assembly being used to scrape material off a moving surface. Detector devices are arranged in connection with the doctor assembly and are used to monitor the operation of the doctor assembly and, if the doctor member drops through, to perform, together with the paper machine's control devices connected to the detector devices, the measures necessary to run down, before the doctor assembly being monitored, the web being manufactured on the paper machine, in order to eliminate the disturbance caused by the drop-through and to perform repair and/or protective measures. The invention also relates to a doctor assembly for implementing the method.
U.S. Pat. No. 5,021,124 discloses a doctor assembly, in which there are two doctor blades. An excellent doctoring result can be achieved using the disclosed doctor assembly, as individual doctor beams, which can be adjusted together or independently of each other, are arranged for each doctor blade. In addition, the construction permits material doctored off the surface being doctored with the second doctor blade to be removed from between the doctor beams. The space is required especially when the paper web is being run down, if the first doctor blade, so to say, drops through the paper web. In that case, the entire paper web runs between the doctor beams and down into the pulper.
A major problem in the doctor described above, and in others, concerns malfunctions, in which the doctor assembly does not operate as designed. This is because few malfunctions are noticed in time. Generally, the operating personnel notices a malfunction quite by accident. Even a small malfunction, if it continues for a long time, can result in great damage to equipment, particularly in the press section of a paper machine. On the other hand, one significant problem in a modern high-speed paper machine is precisely the second doctor blade becoming blocked, if the first doctor blade drops through. A large amount of material then rapidly collects on top of the doctor beams, and can even bend them. In any event, cleaning the doctor assembly is laborious, despite the washing sprays. A corresponding situation occurs, if the pulper malfunctions and the doctored material collects in the doctor assembly, bending the doctor beams. Both doctor blades can then drop through. Dropping through and a poor doctoring result are also disadvantageous to the operation of doctors located in other positions too. Using the state of the art it is, however, practically impossible to detect malfunctions and prevent damage.
European patent application number 989234 discloses a doctor, which is used to scrape a press roll. In this doctor, the angle of the blade holder or the doctor blade is monitored by means of sensors, which are connected to the control of the pick-up roll. For example, a sudden change in the angle of the blade holder is detected as a malfunction, in which case the pick-up roll is raised to avoid damage to the press. However, it is impossible to use the arrangement disclosed to detect a drop through, or other malfunctions of the doctor.
In a drop through, there is, after the doctor blade, material on the surface being doctored, which remains unnoticed when monitoring the angle. Thus, the assembly disclosed is mainly only suitable for detecting foreign bodies and random accumulations of material on the press. The assembly will also easily give false alarms, if it is set to detect very small changes in angle or position. Correspondingly, if the assembly reacts only to large changes, it will be too late to prevent damage.